Thin film bulk wave acoustic resonators (FBAR) for wireless applications

For some time, FBAR technology has lagged behind ceramic technology and surface acoustic wave resonator (SAW) technology for commercial applications. There were several technologies that had to be developed before FBAR technology became viable for rf filters. First, a process is needed that can make the resonators manufacturable, robust and repeatable. Second, maximizing the coupling coefficient, k_t ² and the Q of the resonator (k_t² Q product) is necessary. Another technology needed is a method to eliminate ripple (or "suck out") associated with lateral mode excitation. Lastly, a method is needed for maintaining a uniform thickness (for frequency control and a means to target frequency to within +-0.03%). If one overcomes these sets of hurdles, the rewards are high. The Quality factor, Q, inherent in these structures is impressive (over 2500) and the intrinsic k_t² has been inferred to be close to the theoretical maximum of 6.5%. The k_t ² Q product (Figure of Merit for FBAR filters) have been as high as 100 for our devices. These two properties can be combined in a filter to achieve low pass band insertion loss and extremely sharp skirts. One intrinsic advantage of FBAR over SAW technology is the ability to handle input power in excess of 4 Watts. Resistance to Electrical Static Discharge (ESD) is another desirable property of FBAR devices. Finally, FBAR technology is intrinsically a "low temperature" process technology-compatible with semiconductor technology. This implies future integration of FBARs with semiconductor circuits